DISSOCIATION CURVE. 137 



The calculation of the percentage saturation of haemoglobin 

 with oxygen is made by subtracting the first reading from the 

 second, dividing by the second reading and multiplying by 100. 

 Suppose in the observation made at 20 mm. partial pressure of O2 

 the first reading is 24 mm. and the second, 108, then 

 108-24 



77 -% Hb and 22 - 3 % Hb 



The result must now be plotted on coordinate paper with the 

 percentages of HbO along the ordinates and the partial pressure of 

 Oxygen on the abscissae. The experiment should be repeated, 

 using 10 mm. and 40 mm. pressures of oxygen, and the results 

 similarly plotted. By joining the points, the dissociation curve for 

 blood is obtained. Care must be taken to see that the bottle is 

 sufficiently shaken so that the partly reduced blood absorbs all the 

 oxygen and gives it up again with ferricyanide. It is particularly 

 in the latter operation that care must be taken. 



The Influence of Carbon Dioxide in Lowering the Dissocia- 

 tion Curve can be readily shown by the method. The procedure 

 is as follows: After the pressure has been reduced to the desired 

 degree in the tonometer, the latter is placed in a horizontal position 

 so that the blood lies along the walls, free of the ends. A CC>2 

 generating apparatus (Kipp's) or a bottle containing this gas is 

 then connected by suitable tubing with the free end of the tono- 

 meter, care being taken before making the connection, to fill the 

 tubing with CCV To accomplish this a slow stream of the gas is 

 maintained and the air in the tubing beyond the screw clip (1) is 

 squeezed out before connecting with the CC>2 generator. The most 

 suitable partial pressure of CC>2 to work with is 40 mm. and to 

 attain it the C(>2 apparatus is first of all opened and the screw clip 1 

 very cautiously loosened until, with clip 2 open but 3 and 4 closed, 

 the mercury descends about 40 mm. in the barometer. Clips 1 and 

 2 are then tightly screwed down, and the tonometer removed, the 

 further procedure being exactly as described above. 



The effect of the 40 mm. of CO2 will be found in the above 

 example, where a partial pressure of 20 mm. C>2 was used, to reduce 

 the percentage of HbO from 77 to about 35. 



